Is a 4th generation elisa 100% conclusive if done 6 months after sex with positive female?is it possible to miss the virus?

The 4th generation HIV test is very accurate, it has a sensitivity and specificity of over 99.5% (it can both rule out and confirm true positive and true negative tests). The test can miss an infection if collected within the "eclipse period", which is 10 days from exposure to when a test can confirm a positive. More info here: https://www.cdc.gov/hiv/clinicians/screening/diagnostic-tests.html

If you mean 4th gen combo test for HIV, a negative result at 6 months after exposure rules out HIV infection. For info: HIV test window periods https://www.cdc.gov/hiv/basics/hiv-testing/test-types.html